The operational amplifier is one of the most useful analog ICs available, and one of the most daunting for beginners and experienced engineers.
Above is a diagram of an op-amp in a closed loop non inverting configuration. Probably the most used configuration. Lets assume this is an ideal amplifier. R1 is the input and R2 is the feedback. When R1 = R2 the circuit is in unity(meaning input = output).
To calculate output use the following formula.
– 
R3 can be calculated by the following.
That is inverting.
In the above link the gain equation is missing a parenthesis someplace.
I retrace that last post. It was just a printing error on my end.
That is an inverting configuration, and with the + input at ground, you are going to need a split supply (both positive and negative voltages) and for a positive voltage in you get a negative voltage out. I don’t think that is the most common opamp configuration these days, people like single-supply circuits.
The semiconductor companies have produced all the free documentation
you’ll ever need about op-amps, including free books. Just google and read:
“Op Amps for Everyone”, TI
“Handbook of Operational Amplifier Applications”, TI SBOA092a
“Op Amp Applications Handbook”, ADI
The ADI book is available as a dead tree, but it’s also on their site.
Thanks for those recommendations, Jim.